This invention relates to a vehicle control system in which a control unit, sensors and actuators are connected by a transmission circuit.
In modern automotive vehicles, greater emphasis upon electronics has made it possible for such vehicles to be equipped with a large number of electronic devices, and this has resulted in a marked increase in the number of wire harnesses used. Accordingly, in order to reduce the number of wire harnesses used in a vehicle, a transmission system in which a wide variety of signals are transmitted by a single wire harness has been proposed.
Specifically, the specification of Japanese Patent Application Laid-Open (KOKAI) No. 64-36541 discloses a multiplexed transmission system for the body accessories of an automotive vehicle. The multiplexed transmission system is installed in an automotive vehicle and uses a multiplexed transmission circuit to connect a variety of electronic devices provided on the vehicle body. By way of example, these devices include an air conditioner, a car stereo, lamps, wipers and power windows, as well as various meters such as a thermometer for measuring water temperature, a speedometer, etc.
In addition to the above-mentioned electronic devices associated with the body of the vehicle, modern automotive vehicles also come equipped with various electronic devices associated with control of vehicle travel. These electronic devices include an ACS (active suspension), 4WS (four-wheel steering), ABS (antilock braking system), TRC (traction control) and a control device for controlling the traveling state of the vehicle, such as EGI (fuel injection control). It would be desirable to apply a multiplexed transmission system to these electronic devices for control of vehicle travel in the same way as such a system is applied to the electronic devices associated with the vehicle body.
The reason for this is that each of these control devices for vehicle travel, such as the ACS or 4WS, is equipped with its own control unit, various sensors necessary for control, and actuators which undergo control. Since these devices use many more wire harnesses than the electronic devices for the body system, it would be particularly advantageous to reduce the number of associated wires by applying a multiplexed transmission system to these devices. For example, in a case where the same sensor is required by two different devices for controlling vehicle travel, the practice in the prior art is to connect this sensor directly to the control units of the respective control devices for vehicle travel. However, if a multiplexed transmission system is applied to a plurality of such control devices for vehicle travel, it can be so arranged that the sensor is connected solely to one of the control units, with the signal produced by the sensor being applied to the other control units from the first-mentioned control unit via a multiplexed transmission line (i.e., a bus). This is advantageous because it eliminates the need to connect the sensor to each of the control units individually.
A system of the kind shown in FIG. 10 is conceivable as one example in which a multiplexed transmission system is applied to a plurality of the control devices for vehicle travel mentioned above.
In this system, control units 6, 8, 10, 12 of various control devices for vehicle travel, as well as a combination switch unit 2 and a meter unit 4, are connected to a bus 14 serving as a multiplexed transmission line, and various sensors and actuators (solenoids, a flowrate control valve, a cut-off valve and an injection driver) are connected to these control units 6, 8, 10, 12. Furthermore, in a case where a sensor which outputs a signal necessary for a certain control unit (hereinafter referred to as C/Ua) is connected to another control unit (hereinafter referred to as C/Ub), the output signal from this sensor is inputted to the other control unit (C/Ub) via the bus without being connected to the first-mentioned control unit (C/Ua). For example, the arrangement of FIG. 10 is such that a prescribed switch signal necessary for control produced by the meter unit 4, a steering-angle signal produced by a 4WS control unit 6, and a throttle-opening signal and gear-position signal produced by an EGI control unit 12, are inputted to an ABS/TRC control unit 8 via the bus 14.
A problem with the multiplexed transmission system shown in FIG. 10 is that when a certain control unit (C/Ua) develops a fault, another control unit (C/Ub) which receives its input signal via the faulty control unit is no longer capable of receiving this signal. For example, if the 4WS control unit 6 fails, the steering-angle signal applied to the ACS control unit 10 and ABS/TRC control unit 8 from the control unit 6 via the bus 14 is no longer capable of entering the units 8 and 10. This is an impediment to ACS control and ABS/TRC control.
Further, it is desired that a 4WS control program for back-up be stored in the ABS/TRC control unit 8 along with an ABS/TRC control program, by way of example. Then, in the event that the 4WS control unit 6 should fail, 4WS control can be backed up by the ABS/TRC control unit 8. However, with the system shown in FIG. 10, the actuator (solenoid) that participates in 4WS control is connected to the 4WS control unit 6. Consequently, even if the 4WS control program for backup is stored in the ABS/TRC control unit 8, back-up control will be impossible if the 4WS control unit 6 fails. This is another problem that arises in the prior art.
Accordingly, there is demand for a multiplexed transmission system which solves the aforementioned problems through an arrangement that is as inexpensive as possible.